MICROORGANISMS IN THE ROOT ZONE IN RELATION TO TEMPERATURE

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Abstract
A study was made of the numbers and kinds of microorganisms in the root zone of wheat and soybeans grown in the greenhouse at three ranges of temperature: 55–60°, 70–75°, and 85–90 °F. Numbers of bacteria in the rhizosphere and rhizoplane of wheat decreased as the temperature increased, whereas numbers in the root-free soil and on soybean roots increased with increased temperature. The same relationships held for methylene blue reducing, glucose-fermenting, and ammonifying bacteria and those requiring amino acids for optimal growth. Generic analysis of bacterial isolates showed that the percentage of Gram-negative rod-forming organisms increased with a rise in temperature in root-free soil and on soybean roots but decreased on wheat roots. On the other hand, the incidence of Gram-positive and pleomorphic organisms decreased in soil and on soybean roots with higher temperature and increased on wheat roots.Fungal isolations from washed root segments showed a greater incidence of Mucor, Rhizopus, Rhizoctonia, and Gliocladium on soybeans at the high temperature whereas species of Fusarium and Cylindrocarpon were more prevalent at the low temperature. With wheat the most striking feature was the predominance of non-sporing dark species at the high temperature and of non-sporing hyaline types at the low temperature.The number of soil nematodes decreased in the rhizospheres of both plants with increased temperature, the effect being more pronounced with soybeans. With this crop all except one of the genera recognized decreased in number, whereas with wheat, the incidence of certain nematodes such as species of Helicotylenchus, Boleodorus, and Aphelenchoides did not change with temperature.The results suggest that temperature exerts a direct effect on the microorganisms in the root zone and an indirect effect by influencing plant growth.